This invention relates in general to the field of clinical chemistry, and more particularly to the analysis of biological fluids for the presence or absence of certain biochemical components.
One common method for such analysis involves mixing a precisely measured small volume of liquid analyte sample with a precisely measured small volume of one or more liquid reagents, incubating the mixture, measuring the optical absorbance of the incubated mixture, and comparing it with the optical absorbance of the sample measured prior to mixing with reagent.
Another method of analysis involves, instead of optical absorbance measurement, the measurement of fluorescent radiation resulting from flash-type monochromatic irradiation of a fluorescent analyte sample. Absorption of radiant energy by the molecules of the analyte raises the vibrational level of such molecules from the ground state to one of the excited electronic levels. The absorption step occurs within 10.sup.-15 seconds, and the fluorescence results from the spontaneous radiative transition that occurs when the molecules of the analyte return to the ground electronic state upon termination of the incident radiation. The resulting fluorescent light is given off equally in all directions at a wavelength different from that of the exciting light, and with an intensity which is orders of magnitude lower than that of the incident light.
In carrying out analyses by the foregoing methods, the known art utilizes various combinations of precision liquid pipetting devices, pumps, mixers, light sources, light detectors, cuvettes and pipette cleaning means. Sample and reagent volumes are typically measured in precision pipetting devices and are mixed in a mixing receptacle. After incubation of the mixed sample and reagent, as necessary, the pipetting device is used to transfer the incubated mixture to a cuvette for absorbance measurement.
Typical of the general type of precision pipetting device known in the art are those disclosed in Drummond, et. al., U.S. Pat. No. 3,606,086, and Allen, U.S. Pat. No. 3,815,790. Devices of this type include an elongated cylindrical barrel, at one end of which a pipette tube is coaxially mounted. A plunger or piston is axially movable within the pipette tube and is carried by a spring-loaded holder movable axially within and projecting from the other end of the barrel.
Typical cuvettes used in the art accommodate a substantial volume of the liquid analyte whose absorbance is to be photometrically measured. The length of the optical path through such cuvettes is fixed (e.g., 1 cm.), and because only a limited beam passes through the cuvette, only a limited portion of the analyte in the cuvette is actually exposed to the incident beam. Moreover, during such measurement, a substantial amount of the irradiating light is lost through the sides of conventional cuvettes due to scattering. Because the optical path through conventional cuvettes is fixed, if a liquid analyte is too dense to be measured in a given cuvette, either the liquid must be diluted, or a cuvette with a shorter optical path must be used.
Because precision measurement of liquids by pipetting devices requires that liquid adhering to the outer surface thereof be removed, various means have been developed for wiping such surfaces, as by causing the pipette to pierce, or to pass through a hole of smaller diameter formed in, a layer of bibulous material. The procedures thus far developed have not, however, been of a type which is well adapted to automation.